Submarine cable



Aug. 18, 1931. w. s. SMITH ET AL suauanms CABLE Filed Sept, 4; 1928 RswA w immau vPIT I' S a J H6 H9 lnventors every U-C/zaruww, W

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Patented Aug. 18, 1931' UNITED STATES PATENT OFFICE WILLOUGHBY STATHAMSMITH, 0F BENCHAMS, NEWTON POPPLEFORD, HENRY JOSEPH GARNETT, 0FSEVENOAKS, AND HENRY CHARLES CHANNON, OF KENSINGTON,

SUBMAR-INE CABLE Application filed September 4, 1928, Serial No.303,970,. and in Great Britain September 2, 1927.

It has not hitherto been considered possible to construct submarinecables permitting of telephonic communication over very great lengths.The submarine cables between Key est (Florida) and Habana (Cuba) oflength slightly in excess of 100 nautical miles are at present thelongest affording channels for the electrical transmission of speech.Recent improvements in materials for the continuous loading of theconductor may have foreshadowed an increase of this length by two oreven three times, but the possibility of telephonic communication overdistances of the order of, say, 2000 nautical miles has hitherto beenconsidered very remote.

This invention relates to improvements which render possible telephonictransmission over long distances up to about 2000 nautical miles orgreater. v

In the case of telegraphic communication,

v a received current of the order of 10 mi croamperes is suflicient inmost cases to give satisfactory working, and the variation ofattenuation with frequency is relatively of small importance as therange of frequencies required for intelligible signalling iscomparatively small. F or example, the attenuation of a typicalsubmarine telegraph cable is such that a current of 10 microamperes is vreceived when sending direct current reversals with volts at a.frequency of 33.5 (cycles perv second) and satisfactory signalling at aspeed corresponding to this figure is obtained, inspite of the fact thatat twice the above frequency (i. e. at 67-) the received current is only0.2 microampere.

Now for telephonic communication a received current substantiallygreater than 10 'micro'amperes is necessary, as amplification is limitedby the degree to which the system can be balanced for two way working.Further, the band of frequencies necessary for the transmission ofintelligible speech is very much wider than that used in telegraphy,probably extending from 2 or 3 hundred to between 2 and 3 thousandcycles. It is therefore evident that the diminution of received currentrelative to increasing frequency per-' missible in the example given fora telegraph cable is to be avoided in the case of a telephone cable.That is, an efficient telephone cable must have a low attenuation to themean speech frequency (usually taken as 800 or 1000-) and a smallvariation of this attenuation with frequency.

The attenuation per unit length of a cable is given approximately by thewell known where R is the resistance in ohms, L the inductance inhenries, G the leakance inohms, and C' the capacity in farads, allreferred to unit length of the cable. The constants'C and L do notnormally vary appreciably with frequency, and therefore the object ofthis invention is to make the Variation of R With frequency as small aspossible, and further to control G or'to make the term small comparedwith the first term the accom- 13, 1928. The tape is conveniently about.03" wide but this dimension may be considerably varied. The specificresistance of the loading material is advantageously as high as possiblecompatible with the requisite permeability and convenient workability. Aspecific resistance of the loading material of, say, 100 microhms percentimetre cube combined with the thinness of the tape reduces eddycurrent loss to a very small proportion of the total resistance.

As an example, a conductor weighing 1100 lbs. per nautical mile andhaving a direct current resistance of 1 ohm per nautical mile is loadedwith 2 layers of the .001 tape and annealed so as to give an inductanceof 4.5 milli heuries per nautical mile. The eddy current resistance at1000 is 0.015 ohm and at 2000 0.00 ohm per nautical mile. .The result ofapplying the high resistivity loading material in more than one layer of1 mil tape is thus to limit the variation of R with frequency to acomparatively small amount.

The composition of a suitable loading alloy is that described in theco-pending application of TV. S. Smith, H. J. Garnett and J. A. Holden,Ser. No. 230,688, filed Nov. 22, 1927, but other compositions giving therequisite permeability and specific resistance after suitable heattreatment may be used.

lVith regard to leakance, no material has hitherto been found in whichleakance, varies with frequency at a. rate less than in directproportion, and in most cases the variation with frequency isapproximately in ac cordance with the law G== m+ on, where a and b areconstants and a is the It is therefore necessary to reduce the intrinsicvalue of G to as low a value as is practicable.

It has been found that a leakance value of 2 microhnis per cm. cube at1000 at F. can be obtained by using a mixture of gutta percha and/orbalta treated so as to remove all protein, cellulosie matter and otherimpurities. F or this purpose the raw material is dissolved in asuitable solvent, for example, benzene or petroleum spirit, and theimpurities removed by sedimentation and decanta'ion or by filtration, ifnecessary in the presence of known aids to these processes, after whichthe purified material is recovered, for example, by evaporation of thesolvent or by freezing out. In some cases it may be necessary to removethe resin wholly or in part from the gutta percha or other material.This may be done by known methods before or after the purificationdescribed above. The particular composition of the mixture to be used isdetermined by reference to curves connecting leakance and otherelectrical properties, such as capacity, with composition. In certaincases it may be advantageous to add a small percentage of a suitablepreservative, e. g. tannin, to the mixture.

The leakance of the cable will not, however, entirely depend on that ofthe gutta percha insulation alone. The electrical characteristics of thecompound with which the interstices of the loaded conductor are filledbefore covering having been found to aifect considerably the leakance ofthe completed core.

It is a well-known fact that the permeability of many magnetic materialsat low magnetizing forces is greatly diminished by mechanical strain.This behaviour is shown very markedly by many magnetic materials of highinitial permeability such as are suitable for the continuous loading ofsubinarine telegraph and telephone conductors. It has been found thatwhen a conductor, continuously loaded with such a magnetic material ofhigh initial permeability, and covered in the ordinary way with guttapercha, is subjected to hydrostatic pressures of the order of thoseprevailing in ocean depths, the added inductance of the conductor isgreatly diminished, probably as a result of strain produced in theloading material by the outside pressure reducing the permeability ofthe magnetic material.

The present invention further relates to a pressure equalizing mediumwith which the loading material is surrounded before the application ofthe outer covering or insulation, such medium serving as a fluid cushionfor distributing the external pressures evenly over the conductor andthus prevent strains from being set up and thereby reduce this decreasein the inductance of the conductor.

A suitable compound, which does not change its physical condition at thetemperatures and pressures encounter-ml on the ocean bed, comprises asubstantially non-volatile parafiin hydrocarbon, to which rubber, guttapercha, balata or the like may be added as a thickener. In some cases itmay be advisable also to add materials such as resin adapted to increasethe stickiness of the conuiosition.

The parafiin hydrocarbtms used in the preparation of the compound areviscous liquids or semi-solids at ordinary tem 'icratures. The petroleumproducts commonly known respectively as petroleum jelly (soft white oryellow paratlin) and medicinal paraliiu have proved to be very suitablebases for these compounds, and e. g. rubber added until a material isobtained that will coat the copper conductor smoothly and flow into theinterstices of the loaded conductor at ordinary temperatures and at thesame time is sufiiciently viscous not to be displaced from the loadedconductor during the process of covering with the gutta perchainsulation, and that is still sufiiciently fluid at the tenuierature ofthe water ingreat ocean depths to distribute the hydrostatic pressureuni formly over the magnetic material.

Suitableproportions of the ingredients are as'follows: i

The ingredients are mixed together by methods and in machines well knownto those skilled in theart of rubber manufacture. In somecases, where asin Example 2, the proportion of rubber is high, it is advisable to breakdown the rubber by mastication before adding the other ingredients. Theproportions of the ingredients stated above are given merely by way ofexample and should not be regarded as limits.

The advantages of the compounds in accordance with the invention arethat their viscosity can be varied between fairly wide limits to meetany particular case by altering the proportions of the ingredients andalso that their alternating current leakance and their dielectricconstant are relatively low; moreover they have the properties essentialto a pressure-equalizing medium, namely fluidity at low temperatures andhigh pressures, high dielectric resistance, stability of chemicalcomposition and physical state and inertness towards the gutta perchainsulation, copper conductor and loading material.

Owing to their low dielectric constant, these compounds are alsosuitable for impregnating the fibrous insulation of telephone cables.

The. breakdown resistance of these compounds is also exceptionally high,so that they can be used with advantage for impregnating the insulationof power cables.

A cable according to this invention suitable for telephoning over adistance of about 2000 nautical miles may be constructed in thefollowing manner The cable has a twin core, each conductor weighingabout 2150 lbs. per nautical mile and having a direct current resistanceof 0.05 ohm. The loading consists preferably of two layers of 1 mil tapeof permeability 750 giving a total in inductance of 7 milli henries pernautical mile of loop circuit with an effective resistance ofapproximately 1 ohm per nautical mile at 1000 A covering on each core of1100 lbs. per nautical mile of gutta percha prepared as above describedgives a capacity of .2 ,u. F. and a leakance of about 4 microhms pernautical mile of the loop circuit in the laid cable. The attenuationunder these conditions is .003 per nautical mile and the totalattenuation over 2000 nautical miles including head and losses, wouldnot exceed 8 at 1000 Owing to the low characteristic impedance of thiscable (rather less than 200 ohms) and the low hysteresis loss of theloading material, such a type of cable is suitable for higher sendingvoltages than those normally employed, although experiment has shownthat such increase may not be necessary.

lVhat we claim is 1. A loaded signalling conductor suitable for thetransmission of electric currents at audio and carrier frequencies,comprising a conducting member and a layer of loading material having athickness not in excess of about 0.0015 inches and formed of an alloyhaving high specific resistance and a permeability below about 800 atlow magnetizing forces.

2. A loaded signalling conductor suitable for the transmission ofelectric currents at audio and carrier frequencies, comprising aconducting member and a layer of loading material having a thickness notin excess of about 0.0015 inches and formed of an alloy having highspecific resistance and a permeability between 300 and 800 at lowmagnetizing forces.

3. A loaded signalling conductor suitable for the transmission ofelectric currents at audio and carrier frequencies, comprising aconducting member and a loading tape having a thickness of 0.001 inchesand formed from an alloy having high specific resistance and apermeability between 300 and 800 at low magnetizing forces.

i. A loaded signalling conductor suitable 'for'the transmission ofelectric currents at audio and carried frequencies, comprising aconducting member and aloading tape having a thickness of 0.001 inchesand formed from "an alloy having specific resistance of the order of 100microhms per centimeter ing high specific resistance and a permeabilitvbetween 300 and 800 at low magnetizing forces.

0. A loaded signalling conductor suitable for the transmission ofelectric currents at audio and carrier frequencies, comprising aconducting member, and a loading tape having a thickness not in excessof about 0.0015 inches and formed of an alloy having high specificresistance and a permeability between 300 and 800 at low magnetizingforces, the whole being insulated with a material comprising guttapercha, treated to remove all protein, cellulosic matter, and otherimpurities.

7. A loaded signalling conductor suitable for the transmission ofelectric currents at audio and carrier frequencies, comprising aconducting member, a loading tape having a thickness not in excess ofabout 0.0015) inches and formed of an alloy having high specificresistance and a permeability between 300 and 800 at low magnetizingforces, an outer cover and a pressure equalizing medium consisting of aviscous paratlin hydrocarbon between said loading tape and the outercover.

8. A loaded signalling conductor suitable for the transmission ofelectric currents at audio and carrier frequencies, comprising aconducting member, and a loading tape having a thickness not in excessof about 0.0015 inches and formed of an alloy having high specificresistance and. a permeability between 300 and 800 at low magnetizingforces, the whole being insulated with a material comprising guttapercha, treated to remove all protein, cellulosic matter and otherimpurities, and provided with a pressure equalizing medium comprisingmainly petroleum jelly between said insulation and loading tape.

9. A loaded signalling conductor suitable for the transmission ofelectric currents at audio and carrier frequencies, comprising aconducting member and a layer of loading material applied thereto and inwhich said loading material is surrounded with a congealed or viscousparaliin hydrocarbon which serves as a fluid cushion to distributeexternal pressure.

10. A loaded signalling conductor suitable for the transmission ofelectric currents at audio and carrier frequencies, comprising aconducting member and a layer of loading material applied thereto, andin which said loading material is surrounded with a congealed or viscousparai'iin hydrocarbon comprising petroleum jelly which serves as a fluidcushion to distribute external pressure.

11. A loaded signalling conductor suitable for the transmission ofelectric currents at audio and carrier frequencies, comprising aconducting member and a layer of loading material applied thereto and inwhich saiu

